1. Field of the Invention
The invention is generally related to laser systems and, more particularly, to measurement techniques and apparatuses of laser systems that are used for material processing.
2. Description of the Prior Art
Laser tools are commonly used for material processing in the manufacture of micro-electronic circuits. For example, in integrated circuit fabrication processes laser tools are employed for such material processing applications as ablation, deposition, impurity implantation, radiation induced chemical processes, hardening, and annealing, to name a few.
Controlling the intensity and uniformity of the laser beam impinging on the work surface material is particularly important in delicate operations such as etching down to a specific layer in multilayer device, while leaving the underlying layers undisturbed.
A wide variety of techniques for measuring and/or adjusting laser intensity are known. U.S. Pat. No. 4,459,986 to Karaki discloses a technique for measuring laser energy output from a surgical tool. In operation, a partially transmissive mirror located near the work end of a flexible light guide allows a portion of the laser energy to impinge on a heat sink. A thermocouple senses changes in the heatsink and produces a signal that is used to globally adjust the laser energy from the source. Although this technique measures laser energy at the output, it fails to offer a solution to accurately measuring either energy or beam uniformity at the work surface. U.S. Pat. No. 5,134,273 to Wani et al. discloses globally controlling the output energy of a pulsed light source wherein a portion of the laser light is sampled with a photodetector. The signal from the photodetector is then used to adjust a high voltage power supply that the fires the laser. U.S. Pat. No. 4,940,508 discloses a laser ablation tool wherein a computer controlled attenuator globally adjusts the laser energy impinging on a work piece by inserting different combinations of attenuator plates in the path of the laser beam. U.S. Pat. No. 4,561,721 to Keilmann et al. discloses the use of a wire mesh for laser attenuation. Different mesh structures are inserted into the path of the laser beam, preferably at an angle different than 90.degree. to avoid reflecting energy back to the laser supply, to attenuate the laser energy independently of temperature and of the wavelength of the radiation. While these methods globally control or alter the energy of the output beam, they do not provide mechanisms to accurately measure and finitely control the energy of the beam impinging on the material.
A wide variety of techniques for improving the uniform beam intensity spatially across a beam, often referred to as homogenizing, are known. U.S. Pat. No. 3,670,260 to Koester discloses the use of a wedge type homogenizer in an optical beam forming device. In operation, the wedge type homogenizer divides a beam into a plurality of extensive zones, and then the light is diffused in each zone. U.S. Pat. No. 3,997,240 to Kebabian discloses an optical system that provides a uniform angle of illumination to an interference filter. Kebabian contemplates disposing a focusing means a focal length from the output of a homogenizer and using the focusing means to collimate the incident light into the interference filter. U.S. Pat. No. 4,475,027 to Pressley discloses the use of segmented cylindrical optical elements in an optical beam homogenizer. These elements are difficult and expensive to produce. In addition, they are difficult to align. U.S. Pat. No. 4,793,694 to Liu discloses homogenization of a laser beam by first seperating a central portion of the beam from the two edges, and then combining the central portion with the side portions using mirror pairs disposed along the axis of the laser beam. U.S. Pat. No. 5,109,465 to Klopoteck discloses a beam homogenizer which relies on a complex cylindrical elongated light transmissive wave guide. Klopotek relies on a reflective technique (total internal reflection) that requires complicated angular alignment. While these methods are sufficient for some applications in laser material processing, many advantages are gained by employing reflective objectives, and these methods are incompatible with such objectives.
Many prior art light transmission systems employ fiber optic bundles. U.S. Pat. No. 3,207,034 to Harter discloses a periscopic sight for a submarine which includes fiber optic bundles which are converted from a square configuration to an elliptical configuration, and from the elliptical configuration back to a square configuration. U.S. Pat. No. 4,170,400 to Bach et al. discloses the use of a fiber optic bundle for wide angle cameras and projectors. The wide angle image is converted to a non-distorted rectangular form using fiber optic bundles that have one face in the form of a partial annulus and the other face in a rectangular shape. U.S. Pat. No. 4,530,565 to Markle discloses a fused silica optical transformer that has a circular or rectangular shape at one end and an arcuate shape at the other end. Markle employs curved strip waveguides instead of optical fibers. U.S. Pat. No. 4,932,747 to Russell et al. shows a fiber bundle homogenizer for an excimer laser where the output face is custom shaped for a specific illumination pattern.
Excimer lasers are often used for small area ablation and deletion of surface and sub-surface materials. Leaders in the industry have struggled to produce devices which will improve the uniformity of the surface profiles from these ablations. Poor uniformity is due to non-uniformity of the laser source, surface material interaction, increases in incident energy and many other factors. While attempting to improve beam uniformity, it is also difficult to control the laser light angularly to prevent losses in resolution or energy.
Reflective optics have been employed in prior art laser systems. U.S. Pat. No. 4,749,840 to Piwczyk discloses a laser irradiation system which includes a reflective objective, a light source, and an eyepiece, whereby materials on a work piece are ablated while viewing the work piece. In Piwczyk, split lenses are used to divide and recombine the beam to circumvent the central obscuration of the reflective objective, thus allowing higher energy transmission through the reflective optic and better uniformity.